The use of high purity 2,2-bis(4-hydroxyphenyl) propane, herein referred to as bisphenol-A, as a reactant in the preparation of subsequent formulations such as the preparation of polycarbonate resins is well known in the art. One method of producing biphenol-A is that of reacting acetone with phenol in the presence of an acid catalyst such as hydrochloric acid. In such a reaction, two moles of phenol react with one mole of acetone to produce one mole of bisphenol-A and the water of reaction and acid catalyst are then distilled off to thus produce a completed reaction mixture. In those cases when the completed reaction mixture has phenol in sufficient excess over the initial two-mole reaction requirement or when additional phenol is added to yield a completed reaction mixture wherein phenol is present in at least a ratio of one mole phenol to one mole bisphenol-A, an equimolar adduct of bisphenol-A and phenol will form upon cooling said completed reaction mixture to about 40.degree. C. An improved method of recovering the bisphenol-A from reaction mixtures containing it or an adduct thereof is the subject of the instant invention.
In U.S. Pat. Nos. 3,919,330 and 3,972,950, methods for recovering bisphenol-A from impure mixtures are described. The method with which the patents deal involves dissolving the crude bisphenol-A in a polyhydroxy compound, e.g., ethylene glycol, then adding a certain amount of water, whereby the desired material is precipitated in pure form and finally recovered. These patents each state that East German Patent specification 53,374 crystallizes bisphenol-A from a methanol solution by the addition of water or by the addition of halohydrocarbons, preferably dichloroethane, plus water, followed by washing the crystals. Chemical Abstracts, entry 66, 38331 j (1967) describes another procedure in which bisphenol-A is heated under a vacuum to remove free phenol, then dissolved in methanol, treated with activated carbon, filtered and finally water is added to the filtrate to give purified bisphenol-A. The U.S. patents and the abstract are incorporated herein by reference.
It is noteworthy that in none of the cases was the bisphenol-A in the form of an equimolar adduct with phenol and in all of the cases the water was added to the solution of crude bisphenol-A instead of vice-versa. It has now been surprisingly discovered that the adduct can be broken and the bisphenol-A can be readily obtained in higher than expected purity, regardless of the order of contacting the solution with water and, as a second discovery, that the addition of the solution of either bisphenol-A or the adduct to water provides highly pure bisphenol-A in the form of easily handled rhombic crystals.